Due to deaths while instance boosting at low levels, a few rare cases of quests not credited to all toons and whatnot, there was a significant level difference between my paladins (and also between my shamen). It seems that for both teams my 'main' is 4/5 of a level ahead of the lowest alt. So for a fifth of a level or so, everyone is together. Most of the time, there are some toons a level higher than the rest. It doesn't stop me from questing. It doesn't really hurt at all. Once that little gap starts though, it doesn't go away on its own. Even with extra gathering quests on the 'lowbies' they never did catch up all the way.

*edited typo*